Current transformer



1,640,554 Aug. 30 1927- PETERS CURRENT TRANSFORMER Filed Oct. 1. 1925 WITNESSES: lNVENTOR John F Pefers.

Patented 30, 1927.

. UNITED Y STATES 1,540,554 PATENT OFFICE. a

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Application inea October 1, 1925. sewn in. 89,770,

One object of my invention is to provide a current transformer that shall automatically compensate for errors due to themagnetiz 'ing currents necessary to energize itself.

Another object of m invention is' to provide a current trans ormer that shall be,

free fromthe errors generally caused by reason of current losses in the"secondary windings due to the transformers own mag-v netizing currents.

- In current transformersthere is a dif ference between the theoreticalratio of the primary ampere-turns to the secondary ampere-turns, or unity, and the actual obtained ratio of the primary ampere-turns to the secondary-ampereturns'. This difthe theoretical secondary currentand i's'due to the absorption of a portion of the theostantial financial loss tothe generating comp y- V l ,12

I propose to minimize the lossdue to such magnetizing currents byconstructing"a cur-, rent transformer comprislngmam and aux1l-' iary units, each of which comprises a spe:

cially designed burden so connected that the. current traversing one of'the'bu'rden's isfnotf afiected by the magnetizing' currentsof the" This result is we brought about by constructing the two core sections sorthatthey require the same mag two transformer units.

netizing current and? so c'onnectingffthem the accompanyingdra The single figure isa of the c' cuits and apparatus embodying my invention. l

My inventlqn compr ses, 1n general, a

rimary 1 of the throughtype having two sections 2- and 3 that are associated with and auxiliary secondary numbered 11; that of thejjunction common absorption of a certai-n amountofl they-pri- 7; ,mary cu rent b the core member 8 for the,

by resulting..errors in the desirediratioand phase angle of l secondaryf current of the windings 4 and 5, respectively, that are, in

Lturn,"connected to mam and auxiliary burdens 6 and 7, res ivelyi Coremembers 8 and 9 are associated with the secondary windings i and 5, respectively. A y

s The main burden 6 is connected across the main secondar winding 4 and-generally constitutes a win ing ofa suitable measuring instrument, such as an ammeter. The auxiliary burden 7 is connected across both secondary coils 4 and 5 in series'relation and comprises both resistance and 'reactance characteristics. For ease of description, the

point of connection common to the burden 7 the burden 6 and the secondary winding 4 is to the burden 6 and the secondary windings 4: and 5. is numbered 12 andthat-common to the secondary winding 5 and the common 20 ference is indicated by a current 1055 from v burden 7 islnumbered 13;] y

main burden 6 correspond to the ordinary retical secondary current by the transformer core in order to magnetize itself. Where the 1 25 error due to theratio between the primary ampere-turns and the secondary. ampere: turns is large,'as in current transformers, used in modern generating stations, the ratio of the measured current to the true primary 30 current is suificiently in error to cause' sub measuring circuits commonly employed with instrument transformers, in which the ourturns of the section'2 of the primar'y winding and the number of turns o'f the secondary winding 4." Thiserror is occasioned by the purpose of ,inagnetizing that member; The e f t O t 19 f'apar f the-pr current tothe secondary, winding 45 1s shown transformer. a s

f Thejs'econdary winding, 4,, is given a proper number ofturnste supply the, burden 6 vwith theldesired number of am eres "for its rated current ratio and to supp y an p I ,additional current for neutralizingthe eflect relative to oneof the burdens that the ma 3 '45 netizing current of oneunit cancels the magnetizing current of the other unit.

scribed. The secondary voltage of themeasdgs, uring circuit is thatvoltage' requiredtocause agrammatic view'fthe rated secondary current of the circuit to traverse the burden 6." The magnetizing current for core member 8, expressed in terms ofthesecondary turns, is dependent upon the voltage across the secondary wind-,1

, former.

traversin impedance of, and the current traversing,

the burden 6, it follows that the magnetizing current for core member 8 bears an inalirec6t relation to the impedance of the bur- In order to compensate for the magnetizing current for the core member 8, a second or auxiliary circuit, comprising the secondary winding 5 and the auxiliary burden 7, is provided in which the magnetizing current for the core member 9 is given the same value and the same phase angle as that of the main core member 8 When expressed in terms of their secondary windings. The impedance of the auxiliary burden 7 is such that the voltage across it is always sufficiently greater than the voltage across the main burden 6 so that the magnetizing current of core member 9, in terms of its secondary-turns, will be equal to that of core member 8, in terms of its secondary-turns.

While the magnetizing currents actually are components of the primary current of the transformer, nevertheless, 1t is customary engineering practice to consider them in terms of secondary-turns of the trans- In order to have the same hase-an le in the core members 8 and 9, it is desirable that the shall be of the same grade of iron and shal have the same flux density, and the latter item may be controlled by the number of turns in the secondary winding 5 and the impedance 7; The magnetizing current of the core member 9 may also be controlled independently of the flux density, the grade of iron and the number of seconda -turns in the winding 5, by varying the ength of the core member 9 and thereby varying the length of the flux lines traversing it.

The secondary windings 4 and 5 are so wound u on their respective core members. 8 and 9 tl iat their currents combine at points 11 and 12, so that the difierence in their values traverses the main burden 6. The mathematical values of the two secondary currents are so chosen that the current traversing the secondary burden 6 has a desired ratio to the primary current, which ratio may corres 0nd to a standard commercial ratio for instrument transformers.

Considering the point 11, the current which traverses the auxiliary burden 6 is the algebraic difference between the currents the auxiliary'fburden 7 and the current elivered to that point. Since the current traversing the circuit between the points 11 and 12 is the current delivered by the secondary winding 5, and since the current traversing the circuit t rou h the secondary winding 4 between the points 12 and 11 is the current delivered by the secondary winding 4, it follows that the current traversing the main burden 6 between points 11 and 12 is the difference between the currents delivered by the secondary windings 4 and 5.

The relation between the primary, secondary and magnetizing currents of the transformer are covered by the following simple mathematical expressions where: -I, represents the primary current; I, and I represent the secondary currents in the windings 4 and 5, respectively; I and I represent the magnetizing currents for the core members 8 and 9, res ectively- N and N, represent the ratios of. t 1e secondary to the primar turns for the transformer units associate with primary sections 2 and 3, respectively; and I, represents the current traversing the burden 6. Then Ils n mfl n n la a 2 Lug N ,N, p N,

By making ml n.

1 I L, becomes independent of the magnetizing currents, and may be expressed as:

N"NI D and .. N, N, then from equation (4) Bl "'aoo N, 15 (9) which represents the values for a transformer having a ratio of 15 to 1.

% may be made equal to N2 by several methods. One method is to use the same kind of iron in both of the cores members 8 and 9; design the auxiliary burden 7 so that it has twice the voltage drop across it that Mai exists across the main burden 6 in order that the volta e drop between points 11 and 12 will be tfie same as between the points 12 and 13; have the cross sectional area of core member 9 one third as eat as that of core member 8 because t a secondary winding 5 has three times as may turns as the secondary winding 4; and make iron circuit 9 three times as long as iron clrcuit 8.

Accordingly, a current, the value of which is in excess of the measured current is utilized to combine with the current delivered by the secondary winding 5 whereby the effect of the magnetizing current incidental to the core member 8 is neutralized bythe magnetizing current incidental to the core member 9. i

It will thus be seen that I have provided a current transformer in which, an auxiliary transformer unit and burden are utilized for neutralizing the magnetizing current incidental to the main measuring circuit. Accordingly, the current traversing the measuring instrument is unaffected by the magnetizing current for the core member associated therewith.

It is to be understood that such changes may be made in the arrangement, number and connection of the several parts as shall fall within the scope of the appended claims.

I claim as my invention:

1. In a current transformer, a main unit, an auxiliary unit, a main burden and an auxiliary burden, the auxiliary burden-controlling the direction of the magnetizing currents for the transformer.

2. In a current transformer, a main unit, an auxiliary unit, a main burden and an auxiliary burden, the two burdens being so interconnected with the units that one of the burdens is substantially unaffected by the magnetizing currents in the transformer.

3. In a current transformer, a main unit, an auxiliary unit, a main burden and an auxiliary burden,-the two burdens being so interconnected with the units that the magnetizing currents are substantially neutralized in one of the burdens.

4. In acurrent transformer, a main unit and an auxiliary unit, the secondary windings of which are equally affected by their respective magnetizing currents, a: main burden and an auxiliary burden so interconnected with the units that the magnetizing currents are substantially neutralized in one of the burdens. I

5. In a current transformer, a main unlt and an auxiliary unit, the secondary windings of which are equally affected by their respective magnetizing currents, 'an auxiliary burden for controlling the direction of the magnetizing currents and amain burden, the burdens being so interconnected with the units that the magnetizing currents are substantially neutralized in the main burden.

6. In combination, a main and an auxiliary transformer unit, the primary and secondary windings thereof being connected in series relation, a main burden connected in parallel circuit to the main secondary winding, and an auxiliary burden connected in series relation to the secondary windings.

7. In combination, a main and an auxiliary transformer unit requiring substantially the same, magnetizin currents, the primary andsecondary windings thereof being connected in series relation, with an auxiliary burden for controlling the direction of the magnetizing currents and a main burden connected in shunt circuit to the main secondary winding whereby the magnetizing currents from the transformer units substantially neutralize each other in the main burden.

In testimony whereof, I have hereunto subscribed my name thisi25th day of September, 1925.

JOHN F. PETERS. 

